Oxidation products of α- and β-amyrins: potential tracers of abiotic degradation of vascular-plant organic matter in aquatic environments

Abstract

Environmental contextHow can we know what happens to organic matter in aquatic environments? Although several compounds exist that can be used to trace the origin and state of organic matter, not many are sufficiently stable and specific to trace degradation processes, but α- and β-amyrins can fulfil that role. Such knowledge will help us better understand and better quantify carbon fluxes in riverine and marine environments. AbstractIn order to fulfil the current need for stable and specific tracers to monitor vascular-plant organic matter degradation in aquatic environments, α-amyrin (urs-12-en-3β-ol) and β-amyrin (olean-12-en-3β-ol) were oxidised in vitro and their abiotic degradation products quantified in environmental samples from the Rhône River in France. Although they appear inert to photooxidation, they are clearly affected by autoxidation and the tracer potential of the resulting products was confirmed. Autoxidation of α- and β-amyrins produces urs or olean-12-en-3-one, 3β-hydroxy-urs or olean-12-en-11-one, urs or olean-12-en-3β,11α-diol and urs or olean-12-en-3,11-dione. 3β-Hydroxy-urs-12-en-11-one and 3β-hydroxy-olean-12-en-11-one, the main oxidation products detected, were selected as autoxidation tracers. These compounds, specific to autoxidation, were detected in dry leaves of Smilax aspera and in suspended particulate matter samples collected in the Rhône River and evidenced the importance of autoxidation in the degradation of organic matter of terrestrial origin.

Metal Ions and Hydroperoxide Content: Main Drivers of Coastal Lipid Autoxidation in Riverine Suspended Particulate Matter and Higher Plant Debris

“…Also, because of the set of lipid tracers used, it is clear that the autoxidative processes observed acted on SPM of terrestrial origin [9,10]. It has been demonstrated that autoxidation is induced when particulate matter is incubated in seawater, and this was explained by the potential desorption of metallic ions upon contact with seawater [6].…”

“…Betulin and both amyrins are also unambiguous tracers of terrestrial vascular plants [9,10]. It has been long known that transition metals can catalyze autoxidation in biological systems [7,8], but could there be other drivers in the environmental systems?…”

“…In light of recent observations made in the Rhône River [5] and during in vitro experiments [9,10], we selected a set of lipid tracers allowing us to trace both the origin of SPM and the degradative processes undergone by lipids in SPM.…”

Electron ionization mass spectrometry fragmentation and multiple reaction monitoring quantification of autoxidation products of α‐ and β‐amyrins in natural samples

“…These ubiquitous compounds are typically employed as general tracers of higher plant input during environmental studies. Recently, it was demonstrated that autoxidation (free radical oxidation) of α‐ and β‐amyrins affords 11α‐hydroperoxy‐urs‐12‐en‐3β‐ol ( 5 ) and 11α‐hydroperoxyolean‐12‐en‐3β‐ol ( 6 ), respectively (Scheme ). Compounds 5 and 6 , which are not affected by NaBH 4 reduction which is often employed during the treatment of natural samples containing labile hydroperoxides, are thermally cleaved to the corresponding ketones (3β‐hydroxy‐urs‐12‐en‐11‐one ( 7 ) and 3β‐hydroxy‐olean‐12‐en‐11‐one ( 8 )) during gas chromatography (GC) or GC/mass spectrometry (MS) analyses using hot injectors (splitless) (Scheme ).…”

“…The ketones 7 and 8 were thus proposed as tracers of abiotic degradation (autoxidation) of terrestrial higher plants in the marine realm . Although very high proportions of these tracers could be easily detected by GC/MS operating in selected ion monitoring (SIM) mode in particulate matter and sediment samples collected in the Arctic and tropical zones, their quantification in samples where terrestrial organic matter is present in lesser proportions remains more problematic.…”